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Wolkenbeisser
02-09-2006, 04:10 AM
An Airbus flies at a certain height. It carries 100 doves in a big cage in the cargo-room. Suddenly, all the doves begin to fly around in their cage. And now the question: Is the Airbus now lighter or remains the weight the same?

3, 2 , 1 GO! http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

Markku38
02-09-2006, 04:34 AM
Hmmm.......I say no it doesn't...
I say that it comes little bit heavier http://forums.ubi.com/images/smilies/10.gif

Same happen if elevator has 3 people and there comes fly - when that fly moves his wings it makes air to move and therefore comes more cargo..little bit http://forums.ubi.com/images/smilies/16x16_smiley-surprised.gif

Low_Flyer_MkVb
02-09-2006, 04:45 AM
Don't make me post the bullet and the gun one... http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

tigertalon
02-09-2006, 05:05 AM
Originally posted by Wolkenbeisser:
An Airbus flies at a certain height. It carries 100 doves in a big cage in the cargo-room. Suddenly, all the doves begin to fly around in their cage. And now the question: Is the Airbus now lighter or remains the weight the same?

3, 2 , 1 GO! http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

If a plane is pressure sealed, it still has to carry the birds. It is like having a big pot full of water, in which you put 1kg of wood. Wood will float on watter, but pot will, of course, be heavier for this 1kg.

From physical point of view: In order for birds to fly, they have to produce an amount of flow of air downwards. This flow than applies force DOWN to an aircraft, that is exactly the same as birds weight.

LoBaron
02-09-2006, 05:20 AM
weight stays the same but the pilot will have to constantly retrim the plane when the doves start attacking the passengers...

Wolkenbeisser
02-09-2006, 05:26 AM
tt: You win!

http://forums.ubi.com/images/smilies/11.gif

Ohhoooo.... already solved http://forums.ubi.com/images/smilies/16x16_smiley-sad.gif

jimDG
02-09-2006, 05:34 AM
you put a fly in a jar, put the jar on scales, and wait till the fly stops flying about and dies. if the weight changes - you'll know the airbus answer http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif
and the answer is - mean weight is the same, it just fluctuates more if something is moving (theres acceleration and changing the center of gravity)). its like measuring the weight of a ball on a spring that is oscilating up and down, and then measuring the same when it stops.

SithSpeeder
02-09-2006, 07:27 AM
I think I disagree. Let's change the analogy to 500 lb bombs instead of birds for the purpose of order of magnitude.

At the point where a B-17 drops its load, the whole plane gets lighter and "goes up" assuming the pilot doesn't immediately re-trim the plane. If he had, for example a 100 foot tall fuselage (to better illustrate this point) and he dropped the bombs that were attached to the ceiling, his plane would become effectively lighter until the bombs struck the bomb bay doors that he forgot to open. Whether they fall or fly around a bit inside is irrelevant--they are no longer contributing to the plane's downward force due to gravity. Or instead of a bomb, imagine a rocket that gets released from the ceiling and flies around inside in circles. Upon ceiling release, the plane is lighter. Upon running out of fuel and landing back inside the plane, the plane is heavier.

Or think of a balloon. At the point where he throws sandbag ballasts over the side (so they are in free fall), the balloon goes up. But as soon as the rope that is attached to that ballast that he forgot to cut goes taut, then the balloon goes back down to where it was.

* _54th_Speeder *

Viper2005_
02-09-2006, 07:44 AM
If we forget that the airbus is burning fuel and simply treat its cabin as a control volume then things are actually still non-trivial.

i) the control volume is filled with gas at constant pressure (thanks to the pressurisation system). However, the birds are respiring and therefore the composition of the gas in the control volume changes.

Oxygen is replaced with Carbon Dioxide.

Since pv=nRT this implies that if constant pressure is maintained in the control volume then the effect of respiration will be to increase the mass of gas held within it.

On the other hand, the birds are losing mass in the form of Carbon and Hydrogen in the glucose they're respiring (which their lungs dump as carbon dioxide and water vapour).

The density of humid air is generally less than the density of dry air and so this will act to reduce the mass of gas in the control volume.

In addition, the birds are not 100% efficient in their energy conversion and will therefore raise the temperature of the control volume which will also act to reduce the volume of gas it contains.

The mass of the gas contained within the control volume may therefore be said to be a function of the type of airconditioning employed.

ii)Of course we don't live in a simple Newtonian universe. We must consider the effects of relativity.

This implies a conservation of mass-energy; since e=mc‚¬≤ it is reasonable to assume that the birds' conversion of chemical energy into mechanical work implies a proportional reduction in their rest mass...

Ask a stupid question...

LoBaron
02-09-2006, 08:59 AM
Originally posted by SithSpeeder:
If he had, for example a 100 foot tall fuselage (to better illustrate this point) and he dropped the bombs that were attached to the ceiling, his plane would become effectively lighter until the bombs struck the bomb bay doors that he forgot to open. Whether they fall or fly around a bit inside is irrelevant--they are no longer contributing to the plane's downward force due to gravity.

* _54th_Speeder *

you are confusing weight and inertia. http://forums.ubi.com/images/smilies/35.gif

arjisme
02-09-2006, 09:20 AM
No, the airbus will not take off. http://forums.ubi.com/images/smilies/35.gif

tigertalon
02-09-2006, 09:25 AM
Originally posted by SithSpeeder:
Whether they fall or fly around a bit inside is irrelevant--they are no longer contributing to the plane's downward force due to gravity.

* _54th_Speeder *

Wrong, it is cruical here whether they fall freely or whether they fly around. Again, if they "want" to fly around, they have to produce sufficient airflow downwards to compensate for own weight (like helicopter or rocket). This flow then has to stop somewhere, and finally it will stop at floor of the plane, applying exactly the same force on the ground as is weight of bombs.

SithSpeeder
02-09-2006, 11:17 AM
LoB--

I don't think I'm confusing weight and mass/inertia. Weight of an object is simply the mass times acceleration (force of gravity).


I disagree, TT.

Back to the balloon example. If the "sandbag" was instead a human hanglider, the moment he jumps from the balloon, the balloon would be effectively lighter and go up. Regardless of whether he flaps his wings or rides a thermal or has a rocket strapped to his back, he can fly around all he wants. But when he goes to land back on the balloon, his weight becomes part of the total weight of the balloon. And the hanglider isn't producing "downflow" that stops somewhere--that's not how lift is produced...not for a helicopter or a bird.

* _54th_Speeder *

georgeo76
02-09-2006, 11:31 AM
It's the same weight.

The doves have lost no mass. Their mass is still being supported by their wings, and therefore by the air lifting them up. The air is in the airbus, thus; no change.

rnzoli
02-09-2006, 11:56 AM
Whatever, the pigeons will certainly take off from the conveyor belt!!!! http://forums.ubi.com/images/smilies/16x16_smiley-very-happy.gif

(Sorry for stupid post, but I am bored while waiting fro 4.03m download. http://forums.ubi.com/images/smilies/35.gif

LoBaron
02-09-2006, 04:24 PM
Originally posted by SithSpeeder:
LoB--

I don't think I'm confusing weight and mass/inertia. Weight of an object is simply the mass times acceleration (force of gravity).


* _54th_Speeder *

what i was referring to was that you compared the flying doves with your bomb example. thats 2 different physical effects...